It is generally desirable to optimise the power output by a wind turbine. During periods of high wind shear the power output may be optimised by changing the pitch angle of the blades. Blade pitch control is well known in the wind turbine field. Most commercial wind turbines use collective pitch control in which the pitch of all blades is controlled by changing a common pitch reference. However, it is also known to use a cyclic individual pitch control according to which the pitch of each blade is controlled cyclically as it rotates. The three turbine blades may have a common control which applies a cyclic control to each blade offset by 120°.
It is presently known to optimise power output based on an optimal collective pitch angle which is determined from nacelle wind speed and rotor speed. Thus, the wind speed on which the correction is based is the wind speed as measured at the nacelle. However, the wind speed across the length of the blade and around the plane of rotation can vary enormously. With a standard roughness induced wind shear profile, the wind speed at the top and the bottom of the rotor plane, that is furthest from and nearest to the ground, can vary by +10% to −15% respectively compared to the wind speed as measured at the nacelle. In cases of higher shears, these deviations may be from +30% at the top and −40% at the bottom. In consequence, as the operating pitch set points are determined by nacelle wind speed, the turbine is not operated at optimum set points over the entire sweep of the rotor plane.